In recent years, large area thin-film transistor (TFT) arrays have found important applications in imaging devices and display devices. However, fabrication of arrays of TFTs is relatively expensive due in part to the cost and complexity of the photolithographically-based processing techniques used in the fabrication process.
Recently organic semiconductors have been used to form TFT arrays. Such organic semiconductors are preferred due to their compatibility with flexible substrates. Using organic semiconductors also reduces fabrication costs. One reason for the lower fabrication costs is that solution-processable organic semiconductors can be patterned using jet-printing, screen printing, or micromolding. Jet printing is described in U.S. Pat. No. 5,972,419 “Electroluminescent Display and Method for Making the Same”, WO0146987A2 “Inkjet-Fabricated Integrated Circuits”; screen printing is described in “All-Polymer Field Effect Transistor Realized by Printing Techniques” F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava Science 265 1994 p1684-1686 and micromolding is described in U.S. Pat. No. 6,322,736 “Method for Fabricating Molded Microstructures on Substrates” which are all hereby incorporated by reference.
However jet printing, micromolding and screen printing-based fabrication processes are not without difficulties. Jet-printing of organic semiconductors is a serial process and is thus slow. Also, jet printing of organic semiconductors is difficult because such printing involves special printheads maintained in precise alignment with printed features for extended periods of time. In addition, the size of the printed features is controlled, in part, by the wettability of the jetted solution on the substrate. Wettablility can be difficult to precisely control.
Micromolding techniques also present difficulties. Molding techniques usually call for the mold, frequently formed from an elastomeric polymer, be brought into contact with a substrate over a large area. Good registration of the mold and substrate can be difficult to achieve. Often, elastomeric molds are incompatible with the organic solvents used to dissolve the organic semiconductors. Screen printing is faster than jet printing because screen printing provides a parallel method of depositing the organic semiconductor. However, screen printing is typically incapable of fabricating small features with dimensions less than 100 micrometers due to: spreading of the deposited liquid on the substrate, difficulties in fabricating screens with fine features, and difficulties in registration between the screen and previously patterned features.
Thus a method of patterning small feature sizes with a solution-based organic semiconductor in a rapid, highly parallel manner is needed.